Application of biochar to soils may result in plant contamination and human cancer risk due to exposure of polycyclic aromatic hydrocarbons

Biochars are added to soil to improve agronomic yield. This greenhouse- and field-scale study evaluated poly cyclic aromatic hydrocarbon (PAH) contamination in 35 commercial and laboratory-produced biochars, and assessed the effects of biochar amendment of soils on PAH accumulation in vegetables and the risk for cancer. The total and bioavailable PAH concentrations in biochars varied from 638 to 12,347 mu g/kg and from below the detection limit (BDL) to 2792 mu g/kg, respectively. PAH formation in biochars decreased with increasing production temperature (350-650 degrees C). Root exudates enhanced PAH release from biochars. The total PAH concentrations in eight edible vegetables growing in biochar-amended soil varied according to biochar and vegetables type from BDL to 565 mu g/kg. A health risk assessment framework was integrated with the benzo [a] pyrene toxic equivalency quotient and the incremental lifetime cancer risk (ILCR) to estimate the exposure risk for human beings via ingestion of PAH-contaminated vegetables. The total ILCR for adults was above 10(-6), which suggests a risk to human health from direct exposure to PAHs in vegetables grown in biochar-amended soil. These results demonstrate that biochar application may lead to contamination of plants with PAHs, which represents a risk to human health. The PAH levels in biochars produced using different conditions and/or feed-stocks need to be evaluated and biochars should be pretreated to remove PAHs before their large-scale agronomic application.